1. Field of the Invention
The present invention relates to a surface acoustic wave device having an interdigital transducer disposed on a piezoelectric substrate.
2. Description of the Related Art
A conventional surface acoustic wave device will be now described with reference to FIG. 6.
The conventional surface acoustic wave device 130 includes a piezoelectric substrate 111 made of LiTaO.sub.3, LiNbO.sub.3, or the like, and a pair of comb-shaped electrodes 120 made of Al or the like, disposed opposite to each other on the piezoelectric substrate 111 so as to define an interdigital transducer. Each of the comb-shaped electrodes 120 has a plurality of electrode fingers 122 and a bus-bar 121 to which one of the ends of each of the electrode fingers are connected.
On the piezoelectric substrate 111, a bonding pad 113 is connected to the bus bar 121. This surface acoustic wave filter is accommodated in a package which is not shown in FIG. 6. The bonding pad 113 and a pad (not shown) provided in the package are connected via wire bonding.
In recent years, mobile communication systems or the like operating at high frequency have been advanced more widely. It is required that surface acoustic devices for use in such systems satisfy increasing high frequency requirements.
A surface acoustic wave device is designed based on material factors such as an electromechanical coupling factor, a reflectivity of a reflector, and so forth. These material factors depend on a ratio (h/.lambda.) of the film thickness (h) of the interdigital transducer to a wave length (.lambda.) of a surface acoustic wave which is excited on the substrate. Ordinarily, the ratio (h/.lambda.) of a wavelength relative to a film thickness is limited to the ranges of several percent to ten percent, due to the electric characteristics of the surface acoustic wave device. Accordingly, it is necessary to reduce the film thickness h of the interdigital transducer in accordance with the shorter wave length .lambda. to cope with the requirements for the surface acoustic wave device operating in a short wavelength band, namely, at a high frequency wave band.
However, the surface acoustic wave device is exposed to high temperature conditions during the production steps thereof or during the operation thereof. It has been discovered by the inventors of the present invention, that in such a high temperature environment, defects may be caused in the interdigital transducer such as a small thickness and low strength, due to a difference between the coefficients of linear expansion of the piezoelectric substrate and of the interdigital transducer. Thus, such defects tend to occur in a wide-area portion of the interdigital transducer. That is, such defects are often caused in a bus bar, particularly, in an edge of the bus bar. In the event that a defect occurs in the connection location between a finger of the comb-shaped electrode and the bus bar, the finger and the bus bar are disconnected from each other which causes problems in the characteristics of the surface acoustic wave device, such as increasing of the insertion loss, changing of the frequency band, and other problems.